The present invention combines a porous semiconductor material on a porous substrate with UV illumination (either from solar light or a UV lamp) and an efficient oxidant, such as electrochemically generated ozone (O3), hydrogen peroxide (H2O2) or oxygen (O2), in a method and apparatus for oxidizing organics in water or in air. The invention includes a process and apparatus wherein efficient photocatalytic oxidation occurs in either a two-phase or three-phase boundary system formed in the pores of a TiO2 membrane disposed in a photocatalytic reactor.
In a first two-phase system, a contaminated gas stream (such as air) is passed over a solid, porous semiconductor photocatalyst and a gaseous oxidant (such as ozone, oxygen or a combination thereof) is provided to the porous photocatalyst. In a second two-phase system, a contaminated water stream is passed over a solid, porous semiconductor photocatalyst and a liquid oxidant (such as aqueous hydrogen peroxide) is provided to the porous photocatalyst. In either two-phase system, the oxidant and contaminant sources are delivered over opposing sides of the porous photocatalyst and contact each other adjacent the solid UV illuminated photocatalyst surface to provide oxidation of the organic contaminants. The process and apparatus allow the use of sunlight or artificial light, such as inexpensive low power UV lamps, as the source of UV illumination directed onto the semiconductor photocatalyst surface.
In a first three-phase system, a gaseous oxidant (such as ozone, oxygen or a combination thereof), a liquid containing organic components, and a porous, solid semiconductor photocatalyst converge and engage in an efficient oxidation reaction. Similarly in a second three-phase system, a gas containing organic components, a liquid oxidant, and a porous solid semiconductor photocatalyst converge and engage in an efficient oxidation reaction. In either three-phase system, the pores of the solid semiconductor photocatalyst have a region wherein the liquid phase forms a meniscus that varies from the molecular diameter of water to that of a capillary tube resulting in a diffusion layer that is several orders of magnitude smaller than diffusion layers in the closest known reactors. Optionally, the process and apparatus may be enhanced by allowing the use of sunlight or artificial light, such as inexpensive low power UV lamps, as a source of UV illumination directed onto the semiconductor photocatalyst surface. Furthermore, generation of OHxe2x80xa2 radicals can be enhanced by photolysis of the oxidant (O3 or H2O2) by using UV lamps with a broader UV spectrum that that used in conventional AOPs.
With the small diffusion layer, the oxidant within the three-phase system simultaneously functions as (i) an electron acceptor at the surface of the TiO2 membrane in the photocatalytic oxidation of organic contaminants, and (ii) an oxidant to be photolyzed in the O3/UV (or H2O2/UV) reaction if proper UV illumination is used (i.e., wavelengths below 300 nm, preferably between about 220 and 280 nm). The photocatalytic reactor efficiently mineralizes a variety of organic contaminants at ambient temperature and low pressures.
Another embodiment of the present invention uses a packed bed photoreactor and multiple ways to introduce the oxidant into the reactor. The reactor contains a substrate with a photocatalytic surface, a fluid cell in communication with the photocatalytic surface of the substrate, and a UV light source like that described above. The fluid cell has an ultraviolet transmission surface positioned to expose the photocatalytic surface to ultraviolet light. The substrate can be silica beads which are coated with a photocatalyst such as titanium. The beads are packed into the reactor, preferably around dividers that act as flow fields so that the fluids (contaminant and oxidant) flowing through the reactor are not only mixed but they contact the photocatalytic surfaces on the beads.
The packed bed photoreactor provides greater surface area contact with contaminants thus is able to treat larger volumes of contaminated fluid. The design is more compact than cylindrical reactors and is capable of maintaining a saturation value of oxygen (or other oxidant) throughout the reactor, which promotes greater contact between the contaminated fluid and the photoactive surface and subsequent oxidation of the contaminants.